Friday, September 29, 2006

Faecal sac

Faecal sac

How do birds deal with wastes generated by their nestlings? Those of the Peaceful Dove (Geopelia striata) seen on the left obviously do not practice sanitary hygiene, soiling their nest with their faeces.

Some parent birds actually eat the faeces during the first few days after the nestlings hatch. At this early stage, the droppings are rich in partially digested food as their intestines lack the necessary bacteria for complete digestion.

As the nestlings grow older and the bacteria set in, the faeces need to be disposed of. Many nestlings simply turn around, point their posteriors away from the nest and fire away. Depending on the aim, they may keep the nest clean or end up fouling it.

Others dispose of their waste via faecal sacs. These sacs are made of strong mucous that the parent birds can easily pick and dispose of some distance from the nest. Robins and bluebirds have been reported to fly off and drop the sacs some distance away. Grackles almost always drop faecal sacs over water but when they nest in backyards where there are no rivers or streams nearby, they tend to drop them in swimming pools.

A recent study in Georgia, USA found that Eastern Bluebird (Sialia sialis) places these sacs on electric wires, wooden fence posts, tree branches and even on top of a utility pole. Why? To reduce the chance of predators locating the nests through visual or chemical evidence. Local birders have always been aware that certain species of birds dispose of the nestlings’ wastes via faecal sacs. The image above shows a female Scarlet-backed Flowerpecker (Dicaeum cruentatum) removing a faecal sac full of mistletoe seeds from a nestling. We take for granted that these sacs are disposed of some distance from the nests. But we should keep a look out on exactly where these sacs are disposed.

In November 2004, Tang Hung Bun observed an off-season nesting by a pair of Scarlet-backed Flowerpeckers. He videoed the feeding and removal of the faecal sac which was extra large. This large sac contained green, undigested mistletoe seeds.

Input by YC, images by YC (top, middle) and Tang Hung Bun (bottom).

Link to video provided by Tang here and another videoed by Prof Ng Soon Chye here.